Search for black squirrels begins
Researchers want the public to take part in a project to measure the UK spread of the black squirrel.
It is 100 years since the first of these appealing, but invasive, rodents was recorded in the UK.
Black and grey squirrels are the same species, so the project also aims to find out if these darker, less common squirrels carry the "grey squirrel pox" which infects the UK's red squirrels.
Scientists from Anglia Ruskin University are co-ordinating the study.
All black and grey
- Between 1876 and 1929, about 100 grey squirrels were introduced to more than 30 different sites across England, Wales, Scotland and Ireland
- There are now more than two million grey squirrels in the UK
- Black and grey squirrels are the same species and can interbreed. A mutation in the gene that governs coat pigmentation is responsible for the black colouring
Lead researcher, Helen McRobie explained that the first "official UK sighting" of a black squirrel was in 1912.
It was spotted in Bedfordshire, which was the location of one private menagerie from which the entire UK black squirrel population was thought to have been released.
"Just a few black squirrels were released in one location, whereas the grey squirrels were released in several locations all over the country," explained Dr McRobie.
She added that the rise of the black squirrel had been hard to trace because, as the grey and black populations spread, "they joined up".
So far, black squirrels have only been spotted in Bedfordshire, Hertfordshire and Cambridgeshire.
"Although we know black squirrels are spreading, as yet we don't have evidence that they are living elsewhere in the British Isles," Dr McRobie said.
"We want to understand if the blacks are, in fact, spreading faster than the greys."Roadkill DNA
The team also hopes to build genetic profiles of both black and grey squirrels.
"What I would really love to get from this project would be DNA samples," Dr McRobie said.
At the moment, the main source of squirrel DNA for the the Cambridge-based researcher is roadkill.
"If there's anyone involved in culling or pest control - of either the black or the grey squirrels - it would be fantastic if I could take DNA samples from the squirrels they catch," she told BBC Nature.
The team hope to use these new samples to reveal the genetic secrets of two of Britain's most successful invading mammals.